Objective:

Conventional lithography (the means by which integrated circuits and memory
devices are produced) requires over 1 kg of organic solvent and aqueous waste
to produce a 2-g chip. Carbon dioxide can be utilized as an environmentally
responsible alternative for aqueous and organic solvents in a variety of processes.
Previous research in our laboratory has shown the utility of CO2 in the lithographic
process at 193 nm. However, industry continues to use smaller wavelengths to
decrease feature size and increase performance, with the next lithographic wavelength
being 157 nm. The objective of this research project is to capitalize on our
success with 193-nm negative tone images in CO2 by developing a 157-nm positive
tone image for a CO2 based lithographic system.

In addition to the environmental benefits of switching from aqueous and organic
solvents to CO2, dramatic performance benefits also should be realized. Industry
is using larger wafers, which must be spin coated with a photoresist. By utilizing
CO2, larger surfaces can be coated at lower spin rates and concentrations. Furthermore,
during the development of small images (< 70 nm) with aqueous bases, the
potential for image collapse is very high given the high-surface tension of
water. By replacing water with liquid or supercritical CO2 and a low-surface
tension fluid, the potential for image collapse should be eliminated.

Progress Summary:

The development of a 157-nm positive tone image requires the deposition, exposure,
development, etching, and stripping of a 157-nm photoresist. A positive tone
image is obtained when the exposed region changes solubility such that on subsequent
development, the exposed regions are removed. To adapt this system to CO2, a
157-nm photoresist must be made that possesses liquid CO2 solubility for deposition,
a method for making the material more soluble in CO2 upon exposure to 157-nm
light, and finally stripping of the unexposed resist after etching. Other criteria
include the necessity for low absorbance at 157 nm to allow for complete latent
image transfer, relatively high-glass transition temperature to prevent image
blurring, and etch resistance that will allow image transfer to the substrate.

In the past year, we focused on 157-nm photoresist development following two
thrusts. The first thrust involves the development of a solubility contrast
that can be utilized in CO2. The second thrust centers on the synthesis of a
highly CO2 soluble and optically transparent (at 157 nm) backbone to which the
contrast can be attached.

CO2 is a great solvent for nonpolar fluorinated compounds. So, the design of
the photoresist requires that the material be somewhat polar prior to exposure
and less polar after exposure. Therefore, the chemical reaction that occurs
during exposure must mimic this polar to less-polar switch. As such, we have
targeted the Pinacol rearrangement reaction (see Figure 1 below) as the basis
for our solubility contrast.

Figure 1. The Pinacol Rearrangement Reaction.

In the past year, we have prepared the precursor shown in Figure 1 as a model
to study the rearrangement. Furthermore, we have prepared several polymers (see
Figure 2), which possess CO2 solubility so that upon exposure to acid and subsequent
rearrangement, the solubility change can be examined.

Figure 2. Polymers That Possess CO2 Solubility.

These materials are useful for proof of concept, but the glass transition temperature
(~55°C) is too low. Furthermore, the multiple double bonds in the styrenic
monomer are too strongly absorbing for 157-nm lithography. Therefore, as part
of the second thrust, a backbone structure is being examined, which will possess
a higher glass transition temperature and low absorbance, while maintaining
effective etch resistance and CO2 solubility.

Figure 3. Materials That Possess High-Glass Transition Temperature (140°C
for A and 147°C for B).

The materials shown in Figure 3 have been prepared. These materials possess
relatively high-glass transition temperatures (140°C for A and 127°C
for B). Absorbances at 157 nm are 1.7 and 3.1 mm-1 for A and B, respectively.
Given the fairly high absorbance of B, this material will not be examined further.
Etch resistance of A is promising, with the material etching only 20 percent
faster than the industry standard Novolak. Unfortunately, neither material is
liquid CO2 soluble.

In conclusion, we have prepared model compounds to prove our CO2 solubility
contrast concept. We also have prepared backbone structures that meet our glass
transition temperature and 157-nm absorbance requirements.

Future Activities:

In the near future, we will begin to test our model compounds to prove our
solubility contrast concept. This will involve studying the kinetics of the
rearrangement, both in the small molecule and polymer model compound. Solubility
changes in CO2 will be examined on the model polymer compound. Backbone work
will involve incorporating other monomers, which will maintain the low absorbance
and high-glass transition temperature, while increasing CO2 solubility and improving
etch resistance.

Upon completion of these two thrusts, the two components will be incorporated
together to give us our 157-nm photoresist. This material will then be deposited
via our CO2-based spin coater and imaged with our new ASML 5,000/900 series
193-nm stepper, which should be running by midsummer. This tool will allow immediate
feedback as to conditions that are needed to produce high-pattern transfer.
In conjunction with CO2 development, conditions which minimize image collapse
will more readily be obtained. This will allow rapid extension of this dry lithographic
system to 157 nm.

Relevant Websites:

Progress and Final Reports:

The perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.